The above research project is concerned with a number of aspects of the enzymology of nitrate respiration and assimilation, namely, a continuing investigation at the biochemical and molecular levels of (a) in vitro assembly of assimilatory nitrate reductase by incubation of a cell-free extract of a particular Neurospora nitrate reductase mutant (nit-1), induced by nitrate, and that of other mutants, uninduced wild type and other molybdenum-containing proteins. It includes the purification and charcterzation of both the nitrate-inducible protein of the nit-1 mutant and the presumed molybdenum-containing component (including the ultimate identification of molybdenum-cofactors), (b) Neurospora assimilatory NADPH-nitrate reductase, elucidating its subunit and cofactor interrelationships both structurally and functionally, and its regulatory control, (c) the purification and characterization of the nitrate reductase-related proteins of several nonallelic nitrate reductase Neurospora mutants, and (d) the purification and characterization of the enzymes involved in the stepwise reduction of nitrate to ammonia (i.e., nitrate assimilation) and in nitrate respiration (where nitrate replaces molecular oxygen as the terminal electron acceptor) including denitrification. The experimental material for these investigations are Neurospora (wild type, and several particular mutants), bacteria and tissues of higher plants. In addition to its academic value, the data could be helpful in interpreting and evaluating the relationship between nutritive value and yield with respect to the use of different nitrogen compounds in agricultural practice. It may also prove to have a bearing on the normal and abnormal metabolic states of mammalian tissue in view of the known occurrence of the molybdenum cofactor in mammalian xanthine oxidase, liver aldehyde oxidase, and liver sulfite oxidase.